After oxygen, silicon is the most abundant element in the earth's crust, and it is essential for growth and biological function in a variety of plant, animal, and microbial systems (Schröder, H. C.; Krasko, A.; Le Pennec, G.; Adell, T.; Wiens, M.; Hassanein, H.; Müller, I. M.; Müller, W. E. G., Progress in Molecular and Subcellular Biology, 2003, 33, 249-268). Silicon is found in the form of free silicates (SiO4x−, the salts of silicic acid) and bound silica (a hydrated polymer of SiO2). Silica occurs commonly in nature as sandstone, silica sand or quartzite, wherein it is a hydrated polymer that exist in three different crystalline forms: quartz, tridymite and cristobalite. Of these, only quartz is common. Liquid silica does not readily crystallize but instead solidifies to a glass (Douglas, B. E.; Ho, S.-M. Crystal structures of silica and metal silicates. In Structure and Chemistry of Crystalline Solids, Springer: New York, 2006, 233). Silica is the starting material for the manufacture of ceramics and silicate glasses. In addition it is used as filler in a large variety of applications such as paints, plastics, rubber, adhesives, putty and sealants. In addition, a range of precious stones such as amethyst, agate, jasper, and opal are a build up of silica.
The silicates are by far the largest and the most complicated class of minerals. Approximately 30% of all minerals are silicates and some geologists estimate that 90% of the Earth's crust is made up of silicates. Examples of silicate minerals are feldspar, asbestos, clay, hornblende, and zeolites. On top of this the neosilicates (also known as orthosilicates) present a range of precious stones such as olivine, topaz, and zircon (Douglas, B. E.; Ho, S.-M. Crystal structures of silica and metal silicates. In Structure and Chemistry of Crystalline Solids, Springer: New York, 2006, 233).
Biosilicification occurs globally on a vast scale under mild conditions (e.g. neutral pH and low temperature). In fact, minute planktonic algae (diatoms) control the marine silica cycle and these single-cell plants process gigatons of particulate silica every year (Brandstadt, K. F. Curr. Opin. Biotechnol. 2005, 16, 393 and references herein). In addition to diatoms, also sponges, mollusks and higher plants can carry out biosilicification (Shimizu, K.; Cha, J. N.; Stucky, G. D.; Morse, D. E., Proc. Natl. Acad. Sci. USA, 1998, 95, 6234 and references herein).
The synthesis of silica (biosilicification) is catalyzed by the so called silicateins (Shimizu et al. Proc. Natl. Acad. Sci. USA, 1998, 95, 6234., Zhou, Y.; Shimizu, K.; Cha, J. N.; Stucky, G. D.; Morse, D. E., Angew. Chem. Int. Ed., 1999, 38, 780. Alber, B.; Ferry, J., Proc. Natl. Acad. Sci. USA, 1994, 91, 6909) and as is usually the case in nature, enzymes with the reverse activity (silicase activity i.e. hydrolysis of silica to silicic acid) has been reported from marine sponges (e.g. Suberites domuncula), were the released silicic acid is used by other organisms for making silica skeletons (Cha, J. N.; Shimizu, K.; Zhou, Y.; Christiansen, S. C.; Chmelka, B. F.; Stucky, G. D.; Morse, D. E., Proc. Natl. Acad. Sci. USA, 1999, 96, 361).
Silicase activity has also been reported to be present as an additional activity of an alpha-carbonic anhydrase (Cha et al Proc. Natl. Acad. Sci. USA, 1999, 96, 361), Schroder et al., Progress in Molecular and Subcellular Biology, 2003, 33,249-268, and Muller et all, US Patent Publication No. 2007/0218044.